1. Field of the Invention
The present invention relates to two-stroke cycle (hereinafter two-cycle) internal combustion engines, and more particularly, to small utility two-cycle engines.
2. Description of the Prior Art
Small utility-type two-cycle engines are widely utilized today throughout the world for such purposes as nylon line weed trimmers, leaf blowers, chain saws, small tillers, small generator sets, liquid pumps, jet skis, motorbikes and the like. Such small two-cycle engines are normally piston-ported one-cylinder engines which are gasoline fueled through a carburetor.
Such small utility engines in the field today are serious emitters of hazardous atmospheric pollutants, and they are so widespread worldwide that the cumulative pollutant problem is serious. It is applicants' understanding that in 1999 the state of California will begin to enforce very strict exhaust emission standards for this class of engines, and it is also applicants' understanding that no small two-cycle utility engines manufactured anywhere in the world today will be able to approach meeting these new standards. Other states are sure to follow California's example.
This problem of a disproportionate level of hazardous exhaust emissions from small piston-ported two-cycle spark-ignited engines can only be solved by improved combustion over the entire operating range of the engine. Direct, in-cylinder fuel injection can significantly reduce unburned hydrocarbon emissions by timing the injection of fuel in such a way as to prevent the escape of unburned fuel through the exhaust port during the air scavenging process. However, applicants are not aware of any use of direct, in-cylinder fuel injection in small two-cycle utility engines. This is undoubtedly because conventional electronically controlled accumulator fuel injection systems are prohibitively costly and complex for this type of engine. One such prior art fuel injection system is disclosed in Beck et al. U.S. Pat. No. RE 33,270.
Carbon monoxide (CO) emissions primarily result from inadequate oxygen in the combustion process to create CO.sub.2 exclusively. In a lean combustion process where excess air is available CO is nearly nonexistent. However, a homogeneous lean mixture can result in incomplete combustion, thereby adding to HC emissions. Introducing some degree of charge stratification can resolve problems of incomplete combustion and inadequate oxygen. In this method, a homogeneous, stoichiometric charge is accompanied by a pocket of excess air to complete the oxidation process. This combustion technique is most easily achieved by short duration, high pressure fuel injection. The diesel engine represents a prime example of applying in-cylinder injection to achieve stratified charge combustion and nearly nonexistent CO emissions. Again, however, such fuel injector systems are prohibitively expensive and complex for small two-cycle utility engines. Applicants are not aware of any such charge stratification in any prior art small piston-ported two-cycle utility engines.
NOx (oxides of nitrogen) is naturally low in piston-ported two-cycle spark-ignited engines due to inherent exhaust gas recirculation and resulting low combustion temperatures. NOx therefore does not present a significant exhaust emissions problem in such two-cycle spark-ignited utility engines.
Another emissions problem of small two-cycle spark-ignited engines involves part load operation in which both the fuel flow and the air flow through the engine are reduced in order to maintain an ignitable air and fuel mixture. The air flow restriction, normally created by throttling, results in increased pumping losses and increased brake specific fuel consumption during part load operation. Also, it is normal for crankcase scavenged two-cycle engines to misfire at part load due to the presence of residual exhaust gas, degraded scavenge efficiency and resulting degraded fuel/air ratio control. This part load misfire contributes to added unburned fuel emissions and increased fuel consumption.
Skip-firing or elimination of some firing cycles to provide idle and part load engine operation could solve this part load emissions problem, provided that there is no fuel introduced into the cylinder during nonfiring cycles of the engine. This precludes a carburetor fuel intake and requires fuel injection, with noninjection causing the skip-firing. Such skip-firing was disclosed in the said Beck et al. U.S. Pat. No. RE 33,270, and also in an SAE technical paper, #841094 dated September 1984, Messrs. S. Zazula et al. suggest omission of active cycles in a two-stroke motorcycle engine. They described a sophisticated test setup, but what they described appeared to be nothing more than a laboratory experiment. The aim of the Zazula et al. article is to show the effects of the omission of active cycles in a one-cylinder two-stroke engine on the vibration characteristics of two-wheeled vehicles.
The foregoing includes all of the most relevant prior art of which applicants are aware.